1. Field of the Invention
The present intention relates to a method of producing an optical fiber preform by an OVD (Outside Vapor Deposition) method. Specifically, the present invention relates to the method of producing the optical fiber preform including a soot deposition process by which the outer diameter fluctuation of the optical fiber perform is slight in a longitudinal direction, and the deposition efficiency of the soot (glass particles) is high, and it is possible to adjust a ratio of a diameter of a starting rod portion of the optical fiber perform to a diameter of the optical fiber preform.
2. Description of the Related Art
As a method for producing a large size optical fiber preform at the high speed, there is a method that a plurality of glass particles synthesizing burners are arranged at a predetermined interval opposite to a starting rod, and glass particles (soot) are deposited on the starting rod surface in layer form (multi-layers soot deposition) while the rotating starting rod and row of the burners are relatively reciprocally moved. The relative movement of burners and the starting rod from one end to the other end in the reciprocal movement is called “traverse”. In such the method for depositing the glass particles (soot deposition method), it is important that the outer diameter fluctuation in the longitudinal direction of the glass particle deposit (soot body) is the slightest, and many kinds of methods for that are proposed.
In the multi-layers soot deposition using the plurality of burners, one of factors of the outer diameter fluctuation of the soot body is that a reciprocating speed of the burners and/or the starting rod (that is, relative reciprocating speed of the burners and the starting rod) becomes zero at a turning back position of the traverse (hereinafter, called turning back portion). This causes that an actual soot deposition time at the turning back portion is longer than that on another portion, or the degree of touching of the burner flame onto the soot body is different. Accordingly, when the reciprocate is conducted at a predetermined period, the deposition amount of the soot is varied at the turning back portion, resulting in the outer diameter fluctuation. As the method for resolving this problem, there is a method that the turning back position of the traverse is moved in a predetermined direction for each turn, and after it is moved to a predetermined position, it is moved to a reversal direction and returned to the initial position (Japanese Patent Unexamined Publication No. Hei. 3-228845). Herein, the number of turns for the turning back portion to return to the initial position is called averaged number of turns. With this, the fluctuation at the turning back portion in which actually soot deposition time is extended, or the fluctuation of the degree of toughing of the burner flame onto the soot body, is distributed to the whole soot body, and the actual soot deposition time or atmosphere of the whole soot body is made averagely coincident with each other. Therefore, the deposition amount of the soot is uniform in the longitudinal direction, and the outer diameter fluctuation is reduced.
Further, as a method to reduce the outer diameter fluctuation, based on the method described in the above-mentioned Japanese Patent Unexamined Publication No. Hei. 3-228845, there is a method that the fluctuation of the outer diameter of the whole soot body is measured, and the additional soot deposition is conducted to a portion, in which the soot deposition amount is small, by an auxiliary soot deposition burner so as to reduce the fluctuation of the outer diameter (Japanese Patent Unexamined Publication No. Hei. 10-158025). The fluctuation of the outer diameter of the whole soot body is measured by a CCD camera which can monitor the whole soot body and a central information processor. The auxiliary soot deposition burner can independently traverse the whole soot body.
In a process for depositing the glass particles which becomes a cladding on the starting rod including a core (hereinafter, also described as soot deposition or overclad deposition), the important factor in the characteristic of the optical fiber is that, after the overclad deposition, a ratio of the diameter of the starting rod portion to the diameter of the consolidated and vitrified body (hereinafter, called J ratio) can be accurately controlled to a target J ratio, and there is no fluctuation of the J ratio in the longitudinal direction (Incidentally, a consolidated and vitrified body is synonymous with an optical fiber preform). The fluctuation of the outer diameter in the longitudinal direction, which has been studied in the prior art, means the J ratio fluctuation in the longitudinal direction. In the prior art, it has not been considered that the J ratio is accurately controlled so that it is equal to the target J ratio. In the view of the characteristic of the optical fiber, it is important that the J ratio is accurately controlled to the target J ratio because the characteristic of the optical fiber is changed when the J ratio is changed. Strictly speaking, a diameter of a starting rod differs each other, therefore, it is difficult to attain the target characteristic of the optical fiber when it can not be accurately controlled to the target J ratio. Therefore, it is important in the overclad deposition that minute J ratio adjustment or the deposition amount adjustment to the soot diameter direction can be conducted.
In the case of the method of the above-mentioned Japanese Patent Unexamined Publication No. Hei. 3-228845, when the soot deposition is stopped on the way of the averaged number of turns, the width of J ratio fluctuation is increased. Therefore, it is desirable that the soot deposition is stopped at an integer times of the averaged number of turns. However, when the soot deposition is stopped at the integer times of averaged number of turns, the deposition amount of the soot deposited during one averaged number of turn is quite larger, thereby the minute J ratio adjustment becomes difficult. In the method of the above-mentioned Japanese Patent Unexamined Publication No Hei 10-158025, before the soot outer diameter becomes the target soot outer diameter, the soot deposition by a plurality of main burners is stopped, and then the soot deposition to portions at which the deposition amount is small is conducted by only the auxiliary soot deposition burner so that the target soot outer diameter is adjusted while the outer diameter fluctuation is reduced. However, in this method, the efficiency of the soot deposition is reduced (the necessary time for soot deposition is prolonged). Further, in the method of the Japanese Patent Unexamined Publication No. Hei. 10-158025, it is necessary that a movement mechanism for traversing the auxiliary soot deposition burner over the whole range of the soot body is disposed in a vessel, or the movement mechanism is arranged outside the vessel. When the movement mechanism is arranged outside the vessel, it is necessary to provide a hole for whole range traversing to insert the burner into the vessel.
When the movement mechanism is inserted into the vessel, the impurities are mixed in the soot, and they cause voids at the time of consolidating and vitrifying the soot body into a transparent. When a hole is provided in the vessel, a capability that voids are generated by the impurities mixing from the outside is increased. This is the reason why air inside the vessel is kept in the negative pressure so as not to go out to the outside, because the inside of the vessel is in the acid atmosphere, and thereby the outside air enters into the vessel and the dust of outside the vessel accompanied with it enters into the vessel.